Method and composition for subjecting metals to reducing conditions



amnion AND COMPOSITION FoR-suBmo'riNG METALS T REDUCING CONDITIONS George F. Hulf, Pittsburgh, Pa., assignor to Callery Chemical Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application March 30, 1954 Serial No. 419,909

9 Claims. (Cl. 134-2) An important purpose of subjecting metals to reduc-- 1 ing conditions is to remove oxide films and scale to prepare the metal for subsequent treatment. A common method of accomplishing that is to immerse the metal in a bath of molten caustic soda (NaOH) containing sodium hydride (NaH). The sodium hydride reacts with oxide to develop a clean metal surface. An advantage of that practice is that it is applicable generally to metals that are not attacked by molten caustic soda. A further advantage is that once the oxide present has been reduced there is no pitting or other attack of the metal so that it is not possible to over-treat and thus the procedure is more desirable than acid pickling, which is also used for descaling. However, there are disadvantages in that the treatment must be conducted at a minimum temperature of about 700 F. so that undesirable microstructural changes may in some instances ensue. Also,

'it is necessary to remove the residue carried by the metal by steam or water washing, then acid treating to destroy residual caustic, then water washing to remove acid, and finally drying. Also, NaH treatment is not applicable to metals that are attacked by NaOH.

, It is among the objects of this invention to provide a method of supplying reducing conditions in contact with metals-especially for removal of surface oxide and scale, that is simple, rapid and easily practiced, that is applicable at temperatures 'far below those used in sodium hydride descaling, and that combines other advantages while avoiding or minimizing the disadvantages of NaH descaling.

Another object is to provide novel compositions for practicing the method of the invention. 7

The invention is predicated upon my discovery that its stated objects are attained by contacting the metallic article with a molten mixture of at least two alkali metal borohydrides '(MBH in proportions such that the mixture melts at a relatively low temperature as compared with the melting points of its constituents. In the preferred practice of the invention eutectic compositions are used, and for many purposes it is preferred that one of the constituents of the salt composition be potassium borohydride (KBH As exemplifying compositions in accordance with the invention, reference may be made to the system lithium borohydride (LiBH and potassium borohydride (KBH which has a eutectic melting at 110 C. at the composition 50% by weight each of BBB, and KBH Another composition of low melting point consists, by weight, of about 45% each of LiBH, and KBH and 10% of sodium borohydride (NaBH This approaches the eutectic of this system, and it melts at 96 C.

In the practice of the invention the borohydride composition is heated to a. temperature sufliciently above its melting point that the chilling effect of the metal will be insuflicient to cause solidification of the bath, and the metal article to be deoxidized, or descaled, is then immersed in the bath. The deoxidizing action begins promptly and proceeds vigorously until the oxide has lc Patented May 3,1950,

been completely reduced, when action ceases. For example, when rusty iron is plunged into one of the foregoing melts-at about C. a vigorous reaction between the melt and the oxide immediately occurs with complete method has the further advantage that once the oxide has been removed there is no attack upon the base metal, avoiding a disadvantage of acid pickling procedures.

A further advantage is that in sodium hydride descaling the steam or water cleaning that follows removal from the bath tends to cause reoxidation. In contrast, in the present practice adhering salts may be easily cleaned from the article without danger of reoxidation by means of various non aqueous solvents such as liquid ammonia.

The invention is applicable to the removal of scale from metal to prepare it for further fabricating procedures, such as rolling and drawing, and also to the removal of oxide films from metals to prepare them for receiving various types of protective or decorative coatmgs.

It is applicable generally, moreover, to. practices involving the use of fused salts where reducing conditions are desired, as for low melting heat treating baths'to replace mixtures of nitrites and nitrates whose use is dangerous or impractical for some purposes. And the compositions provided by the invention may be applied also as heat transfer media where reducing conditions are desired.

According to the provisions of the patent statutes, I have explained the principle of my invention and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. In a method of subjecting a metallic article to re-" ducing conditions the step comprising contacting said article with a molten mixture of at least two alkali metal borohydrides, said mixture bein of relatively low melting point compared with the melting points of its constituents.

2. That method of removing oxide from the surface of a metallic article carrying oxide reducible by hydrogen a metallic article carrying oxide reducible by hydrogen comprising contacting said article with a molten mixture of potassium borohydride and at least one member of the group consisting of lithium and sodium borohydrides, said mixture being of relatively low melting point compared with the melting points of its constituents, withdrawing the article from contact with the molten mixture, and removing adhering residue.

5'. A method according to claim 4, in which said molten mixture of borohydrides is substantially,by weight, 50 v percent each of LiBH, and KBH 6. A method according to claim 4, in which said molten" mixture of borohydrides is substantially, by weight, 45 i 3 percent each of LiBH and KBH and the remainder NaBH 7. That method of removing oxide from the surface of a metallic article carrying oxide reducible by hydrogen comprising contacting said article with a molten sub stantially eutectic mixture of at least two alkali metal borohydrides, withdrawing the article from contact with the molten mixture, and removing adhering residue.

8. A new composition consisting essentially of about 50 percent by weight each of potassium borohydride and lithium borohydride.

9. A new composition consisting essentially of, by

V V a A weight, about 45 percent each of potassium borohydride and lithium borohydride, and the remainder sodium borohydride.

5 References Cited in the tile of this patent UNITED STATES-PATENTS 1,724,552 Bellis Aug. 13, 1929 2,377,876 7 Gilbert June 12, 1945 10 OTHER REFERENCES Boron Hydrides and Related Compounds, Schechter et 211., Gallery Chem. Co., March 1, 1951, pages 46-53. 

1. IN A METHOD OF SUBJECTING A METALLIC ARTICLE TO REDUCING CONDITIONS THE STEP COMPRISING CONTACTING SAID ARTICLE WITH A MOLTEN MIXTURE OF AT LEAST TWO ALKALI METAL BOROHYDRIDES, SAID MIXTURE BEING OF RELATIVELY LOW MELTING POINT COMPARED WITH THE MELTING POINTS OF ITS CONSTITUENTS.
 8. A NEW COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 50 PERCENT BY WEIGHT EACH OF POTASSUIM BOROHYDRIDE AND LITHUIM BOROHYDRIDE. 